Method of making an electrically heated protective covering for an airfoil



June 28, 1960 M. B. LUKE 2,942,330

METHOD OF MAKING AN ELECTRICALLY HEATED PROTECTIVE COVERING FOR AN AIRFOIL Original Filed Nov. 26, 1952 4 Sheets-Sheet 1 IN V EN TOR. iza y/ Tzg Luke iffy June 28, 1960 M. B. LUKE 2,942,330

METHOD OF MAKING AN ELECTRICALLY HEATED" PROTECTIVE COVERING FOR AN AIRFOIL Original Filed Nov. 26, 1952 4 Sheets-Sheet 2 IN V EN TOR.

.17 "L Like iffy June 28, 1960 M. B. LUKE METHOD OF MAKING AN ELECTRICALLY HEATED PROTECTIVE COVERING FOR AN AIRFOIL Original Filed Nov. 26, 1952 4 Sheets-Sheet 3 i I I I I i u I INVENTOR. ERMH'E like June 28, 1960 M B. LUKE 2,942,330

METHOD OF MAKING AN ELECTRICALLY HEATED- PROTECTIVE COVERING FOR AN AIRFOIL Onginal Filed Nov. 26. 1952 4 Sheets-Sheet 4 zzvmvron J7'Z 7 United States Patent METHOD OF MAKING AN ELECTRICALLY gp s r pn PROTECTIVE COVERING FOR AN Marvin B. Luke, Los Angeles, Calif., assignor to The B. F. Goodrich Company, New York, N.Y., a corporation of New York Original application Nov. 26, 1952, Ser. No. 322,666, now Patent No. 2,741,692, dated Apr. 10, 1956. Divided and this application Mar. 1, 1954, Ser. No. 413,075

4 Claims. (Cl. 29-155.5)

The invention relates to electrically heated protective coverings including heating structures therefor, and pertains more particularly to methods of making such protective coverings and the individually assembled resistance heating structures therefor, which covering are suitable especially for preventing the accumulation of ice on airfoils and other surfaces in air under icing conditions.

This application is a division of my copending application Serial No. 322,666, filed November 26, 1952, now Patent No. 2,741,692, wherein the electrically heated protective coverings and heating structures therefor are claimed.

Heretofore, electrically heated protective coverings or shoes for mounting on the leading edges of wings, empennage elements and other airfoil surfaces have generally had resistance heating wires extending longitudinally and continuously between opposite, widely spaced-apart marginal portions of the shoe, and the breakage or fracture of one or more of the heating wires, as from the impact of stones, for example, on the shoe has resulted in objectionable loss of heating throughout a substantial area of the shoe. This generally renedered the shoe useless for ice-removal purposes even though it was still serviceable from the standpoint of abrasion and other deterioration.

Prior shoes having chordwise heating wires arranged in series connected groups of wires individually in parallel and electrically connected to spanwise flexible conductors located in the plane of the Wires in uninsulated relation to the Wires to provide spanwise heating zones side-byside in the shoe, have presented the problem of avoiding gaps in the heating along and between adjacent margins of adjacent heating zones. An object of the invention is to overcome the foregoing disadvantages and diiiiculties of the prior shoe constructions for aircraft components.

Other objects of the invention-are to provide for an improved construction of electrically heated protective coverings including electrical heating structures therefor having increased life expectancy and for an improved method of making the coverings including electrical heating structures therefor; to provide for effective ice-removal temperatures at the exposed outer surface of adjacent heating zones of a protective covering suitable especially for the wings and empennage of an aircraft; to provide for continuity and uniformity of heating from one margin or edge to an opposite margin or edge of each heating zone of a protective covering; to provide for uniform or different temperatures at adjacent heating zones, or for uniform or different temperatures per unit of area along a zone, especially a tapered zone, of the covering; to provide for eliminating gaps in electrical heating across the marginal junctions of adjacent heating zones of the covering, especially a covering of tapered shape in plan; to provide for regular or straight-line margins of ad- 'jacent heating zones; and to provide for simplicity, lightweight, thinness, flexibility and smoothness of construction, convenience of manufacture, and for effectiveness of operation.

Still other objects of the invention are to provide for effecting ice-removal temperatures despite the fracture or breakage of one or a plurality of resistance heating elements or wires within a heating structure as by the impact of stones and the like thereon; to provide for electrically insulating all the current-carrying elements one from the other of a flexible heating structure together with electrically insulating adjacent flexible heating structures one from the other; to provide for convenience and accuracy of assembly of elements of the heating structure; to provide for an improved method of making an individual flexible heating structure or unit of rectangular or tapered shape for the covering; to provide for independently assembling each individual heating unit and for independently mounting the same in the covering; to provide for an improved method of making simultaneously a plurality of complementary tapered heating foundation units for the heating structures wherein the 'units are assembled upon a drum and subsequently formed individually into tapered heating structures; to provide for an improved method of joining two or more heating struc tures such that they constitute a single heating section; to provide an improved method of forming a plurality of contiguous unit heating areas in series in a heating structure With marginal conductors therein in adjacent isolated insulated relation one to the other; to provide effectively for extensive flexibility of theheating structure; to provide for continuity of heating from one marginal edge of the heating structure to the other marginal edge thereof; to provide for electrically connecting a plurality of chord- Wise resistance heating elements or wires of the heating structure in parallel and a series of the pluralities of resistance heating elements in series arrangement spanwise; to provide for permanently connecting, electrically and mechanically, one section of a heating structure to another section of the heating structure in series or end to end relation; and to provide for effectively electrically insulating flexible conductors or connectors from adjacent portions of heating elements in overlapping spaced relation to such conductors of a heating structure while maintaining an effective electrical connection of the conductors with the heating elements.

These and other objetcs and advantages of the invention will be apparent from the following description.

In the accompanying drawings which form a part of this specification.

Fig. 1 is a perspective view of electrically heated protective coverings on the leading edges of airfoils of an airplane and constructed in accordance .with and embodying the invention,

Fig. 2 is a plan view from above of one of the protective coverings in its flat as molded condition and of tapered form for mounting conformingly about the leading edge of a wing, for example, of the airplane, parts being broken away,

Fig. 3 is a sectional view in an enlarged scale taken along line 3-3 of Fig. 2, parts being broken away.

Fig. 4 is a sectional view in an enlarged scale taken along line 44 of Fig. 2 and showing a permanent electrical connection between adjacent sections of a heating structure of the covering, parts being broken away,

Fig. 5 is a front elevational view of apparatus for forming a plurality of foundational units used for making heating structures of the invention,

Fig. 6 is an end elevational View of the apparatus shown in Fig. 5,

Fig. 7 is a view like Fig. 5, but in an enlarged scale, and showing a template locating spaced-apart conductors inan initial step of making a foundational unit of a heating structure, parts being broken away and in section,

Fig. 8 is a plan view of the template,

Fig. 9 is asectional. view of, the template taken along line 99 of Fig. 8,

Fig. 10 is a View like Fig. 5, but in an enlarged scale, and showing an intermediate step; of making the foundational unit, with one portion of the drawing indicating the next step in the method of making a heating structure, parts being broken away,

Fig. 11 is a sectional view taken along line 11-11 of Fig. 10, parts being broken away,

Fig. 12 is a plan view from above of the foundational unit in its last step of manufacture and in flat form after its removal from the apparatus, parts being broken away,

Fig. 13 is a sectional view taken along line 1313 of Fig. 12, parts being broken away,

Fig. 14 is a sectional and perspective view in an enlarged scale taken laterally of a heating structure showing the next step of its manufacture using the completed foundational unit shown in Figs. 12 and 13, parts being broken away,

Fig. 15 is a view like Fig. 14 but showing the next succeeding step of manufacture of the heating structure, parts being broken away,

Fig. 16 is a view like Fig. 14 but showing the last step of. manufacture of the heating structure, parts being broken away, and

Fig. 17 is a plan view of the completed fiat heating structure ready for assembling into the protective covering.

Electrically heated apparatus or protective covering or shoes 20 to 30, inclusive, each embodying the invention, are mounted on and conformed to the leading edges of airfoil-s such as wings, empennage, propeller blades and other surfaces of an aircraft 31, as shown especially in Fig. l, for preventing the accumulation of ice on the leading edges in air under ice-forming conditions. The coverings may be adhesively attached to the leading edges as by a suitable liquid rubber cement of the air-curing type, and are electrically connected in any desired manner to suitable control means and a source of electrical heating current (not shown) carried; by the aircraft 31. The construction and arrangement of all the coverings are the same except as to plan form, if desired; consequently only the tapered protective covering 20 extending along an inboard portion of the leading edge of a wing 32 will be described in detail.

The electrically heated apparatus or protective covering 20, shown in Fig. 2 as viewed from above in a flat as molded condition and before attachment to the leading edge of the wing 32, is desirably of tapered shape in plan to accommodate varying thickness of the wing 32 at its leading edge region and is constructed of flexible dielectric material, The dielectric material may be natural or synthetic rubber, or rubber-like material or synthetic material having substantially similar chemical composition or physical properties to natural rubber and to equivalents therefor. Such dielectric material or compositions are included within the meaning of the term rubber-like material used hereinafter in the specification.

The thin sheet-like covering or shoe 20 has electrical heating means or a resistance heating grid 33 located intermediate its thickness and between opposite margins 34, 35 of the covering and constituting a unitary part thereof. The arrangement provides at least one and preferably a plurality of heating zones 36, 37, 38 extending in a direction along a margin, preferably a spanwise margin 34, 35, of the covering with the heating zones in side-by-side adjoining relation one to the other from substantially one end or chordwise margin 39 to substantially the other end or chordwise margin 40 of the covering.

For the particular arrangement shown in Fig. 2 having the elongated tapered form, the frontal heating zone 37 is rectangular i.e. tetragonal' with uniformity of heating per unit of area spanwise and chordwise of the covering, while each rearward heating zone 36, 38 is trapezoidal or tapered i.e. tetragonal in form spanwise with relatively greater width at its one end 39 or inboard end as com- 4 pared to that of the frontal heating zone and having desirably uniformity of heating per unit of area spanwise and chordwise of the covering, the heat dissipation per unit of area at the frontal heating zone being relatively greater than that at each rearward heating zone as for anti-icing I unit of area chordwise and spanwise, relative to one another, or only the rearward heating zones may have the same uniform width and the same uniform heating per unit of area relative to one another. If desired, all tapered heating zones may have gradually decreasing heating per unit of area from the wider or inboard end to the narrower or outboard end of each zone.

The covering comprises an integral inner insulating layer formed or built-up of a continuous base sheet 41 and filler sheets 42, 43 of'suitable thin rubber-like material and desirably of rubbery polychloroprene material or composition known as neoprene which is advantageous, especially because of its resistance to deterioration from heat and aging and its elasticity. of bonded-together sheets 41, 42, 43 the resistance heating grid 33 as a whole is securely mounted. A continuous, thin, reinforcing ply 44 is made of suitable textile fabric sheet material such, for example, as woven or knitted textile fabric of filamentary material of cotton, nylon, rayon, silk, glass fibers or other suitable fibrous material,

' and may be coated on both sides with suitable rubber-like material, preferably rubbery polychloroprene. The rein? forcing ply 44 is mounted on the resistance heating grid 33 and is cemented in place with a-suitable liquid neoprene cement. The reinforcing ply 44, preferably of woven glass fabric, is covered by a continuous, thin outer layer 45 of suitable rubber-like sheet material, preferably rubbery polychloroprene, which is suitably cemented in place upon the ply 44 to provide an aerodynarnically smooth, protective, elastic, outer or weather-exposed surface of the covering 20 highly resistant to abrasion, puncture, aging, oils and gasoline, heat, weather, ozone and sunlight,

The electrical heating means 33 as a whole includes a plurality of individual electrical heating structures 46, 47, 48 in insulated relation one to the other in the heating zones 36, 37, 38. Each heating structure, for example, the tapered heating structure 46, may be of sectional construction, and occupies the entire chordwise width of a heating zone, for example, tapered, zone 36, and extends continuously spanwise along the zone with adjacent margins 49, 50, and 51, 52, respectively, of adjacent heating structures 46, 47 and 47, 48, respectively, in adjoining or abutting and electrically insulated relation one to the other but with the dielectric material at the juncture of the adjacent margins 49, 50 and 51, 52 heated by conduction.

This makes feasible effecting continuity of heating chordwise across all the heating structures 46, 47, 48 and across adjoining margins 49, 50 and 51, 52 thereof, so as to avoid gaps, even narrow gaps, in the chordwise heating especially at the margins 49 to 52. Thus, the invention eliminates aerodynamically objectionable narrow spanwise bands of residual ice remaining on the exposed surface of the outer layer 45 of the covering along and between the adjacent margins of adjacent heating structures, which residual ice produces turbulent flow of air across the wing.

The construction and arrangement of the individual heating structures 46, 47, 48 are the same, except as to chordwise width and except as to the rectangular central heating structure '47 having substantially parallel margins 50, 51 while each of the tapered rearward heating structures 46-, 48 has spanw-ise margins converging towardone another in the direction away from the inboardend 39 Upon the inner layer toward the other or outboard end 40 of the covering. This facilitates the tapered shape of the covering. In view of such similarity, only the construction and arrangement of the tapered heating structure 46 will be described in detail. r

The electrical heating structure 46 as shown especially in Figs. 2., 3, 4 and ,17, is a thin sheet-like body.

The sheet-like heating structure 46 may be formed of a first reinforcing ply 59 and a relatively wider second reinforcing ply 60 in superimposed united relation one to the other to provide inwardly foldable marginal portions 60a and 60b of the ply 60 extending away from the marginal edges of the ply 59. Each ply may be made of straight-laid, square woven textile fabricof suitable filamentary material, preferably heat-resistant woven glass fabric, and may be coated on both sides with unvulcanized, dielectric, rubbery polychloroprene composition. "A continuous, thin layer 61 desirably of rubbery polychloroprene sheet material is mounted for increased insulating purposes on the first ply 59 in overlying faceto-face cemented relation thereto and throughout the area thereof. The second ply 60 has its lateral marginal extensions or portions 60a, 60b arranged in a laterally inward folded disposition in overlapping relation to the rubbery layer 61 and fabric ply 59 and in adhered edgeon contacting relation to the rubbery insulating layer 61 at a face of the body, as shown especially in Fig. 3. This provides dielectric closed fold portions 54, 55 with conductor passages or conduits 57, 58 therein at the span.- wise margins 49, 56 of said heating structure 46, the marginal portions 60a, 60b constituting in part the thin dielectric continuous peripheral walls of the passages.

Preferably, the electrical heating structure 46 has a plurality of flexible, stranded and braided wire conductors 62, 62 and 63, 63 of bronze, copper, copper alloy or other suitable conductive metal and in fiat strip form mounted spaced-apart in succession and in insulated relation one to the other in each conductor passage 57, '58, and secured to and enclosed by the continuous peripheral wall of the passage. This facilitates extensive flexibility of the heating structure by virtue of therelatively short length of each conductor and the independent flexure of a conductor relative to the adjacent conductor in a passage.

For coacting with the conductors to heat the structure, the invention provides a plurality of solid or stranded, resistance heating elements or wires 64, 64 connected at their ends to the conductors 62, 63.

The resistance heating wires 64, 64 may be of nickelchromium alloy or copper-nickel alloy or other suitable metal material and are sandwiched between the superimposed reinforcing fabric plies 59, 60. The wires 64, 64 extend chordwise or across the heating structure from substantially one edge or margin 49 to substantially the other edge or margin 56 with each wire in a transverse plane spaced from adjacent flat faces of the conductors 62, 63 to provide portions of the wires extending across the flat faces in spaced, overlapping, insulated relation to the conductors. The heating wires extend continuously chordwise in closely spaced-apart, substantially parallel, insulated relation one to the other between the spanwise margins 49, 56, and are disposed approximately perpendicular to the longitudinal centerline of the heating structure.

The heating wires 64, 64 continue from the margins in an inwardly folded manner interiorly about the inwardly folded marginal portions 60a, 60b so as to be beneath the outer surface thereof, and about the outer edges of the fiat conductors 62, 63 in insulated spaced relation thereto to a position overlying the other flat faces of the conductors, and terminate at the inner edges of the conductors and such other flat faces thereof. The heating wires extend entirely across such other flat faces of the conductors in direct contact therewith and are soldered by fused metal at 65 to the conductors entirely across said other faces thereof to provide good low-loss electrical connection and secure mechanical connection of end portions'of the heating wires to the flat conductors.

In the preferred construction and for increased insulating and marginal strength purposes, the heating structure has narrow, 2 ply, thin insulating tapes 66, 67 of suitable woven textile fabric coated on both sides with rubbery polychloroprene. The insulating tapes 66, 67 are wrapped once about the flat conductors and over the soldered, folded end portions of the heating wires and are disposed beneath the marginal fold portions 60a, 60b of the reinforcing ply 60 in a manner to completely enclose the conductors and the folded end portions of the heating wires so as to further insulate and protect the conductors and the heating wires, as shown especially in Fig. 3, and to coact for such purposes with the fabric plies 59 and 6t) and the insulating rubber layer 61. This feature of the wrapped around insulating tapes 66, 67 is important to provide both mechanical and electrical strength at the conductor-containing passages of the heating structure and to assure fully enclosing the conductors and folded end portions of the wires in a walled pocket of insulating material.

The construction and arrangement of the heating structure 46 provides chordwise-extending resistance heating wires 64, 64' each desirably of the same cross-sectional area and located in a transverse plane close to the outer surface of the covering for effective heating purposes. The wires 64 are arranged in groups with the number of wires in each group such as to provide the desired substantially uniform heating per unit of area of the heating structure at each group, the individual wires of each group are connected in parallel electrically by the flexible conductors 62, 63, and the flexible conductors extend between and connect adjacent groups so as to place them in series electrically. The number of wires in eachgroup is such that the individual wires thereof vary or differ only slightly in length and resistance relative to one another despite the convergence of the conductors. The number of wires in each group may bethe same for all groups for substantially uniform and equal temperatures per unit of area of all groups, or may vary from group to group, if desired, while providing for substantially uniform temperatures per unit of area of each group. In the preferred construction each group spanwise has the same number of individual wires, for example, 8 wires to provide for substantially equal and uniform heating per unit of area throughout the heating structure even though it is tapered. It will be noted that the invention makes feasible the provision of graduated temperaturesin the heating zones of the covering, chordwise, or spanwise, or both, although the heat dissipation per unit of area of a group of wires is substantially uniform throughout the group.

,A further advantage of the heating structure 46 even though of tapered construction is that normally the' heating wires 64 are individually of relatively short length and each extends only a short chordwise distance across the covering as a whole when a plurality of spanwise heating zones are provided in the covering, so that fracture or breakage of a wire or wires in a group as by impact of stones and the like, effects the heating of only a very small, highly localized area of the covering. This results from having the heating current, which would normally have passed through the broken wires, pass through the remaining wires of the group and through the conductors 62, 63 to the adjacent group where such current is distributed to and passes through all the wires of the adjacent group. However, the remaining wires in the group with a broken wire or wires therein, will have slightly increased temperature rise as compared to that at the adjacent group because of the increased heating current passing through said remaining wires, but this increased temperature rise is highly localized and limited to the particular group, and does not materially affect the remainder of the resistance heating grid 62, 63, 64 of the electrical heating structure 46. The arrangement makesfeasible these new results without requiring the use of supplementary wires across the wires of a group and disposed between the spaced-apart conductors for the group. j

When the covering is of considerable length, the electrical heating structure 46 may be made of two or more independent sections 46a, 46b each of like construction except as to chordwise width and, if desired, spanwise length, for convenience of manufacturing purposes. The adjacent mating ends of the sections 46a, 46b are suitably spliced together as shown especially in Fig. 4, and the adjacent ends of adjacent conductors 63, 63, for example, in a marginal passage of the sections 46a, 4611 are abutted and soldered by fused metal to one another and to an underlying, thin copper or other conductive metal disc 97 in the manner .shown in the drawings. However, the adjacent slightly spaced ends of the other conductors 62, 62 in the other marginal passage of the sections 46a, 46b are electrically and mechanically insulated from one another by an intervening spacer or filler 96 of suitable rubber-like material. This produces the unitary, individual heating structure 46 in its entirety and the opposite end conductors 63, 63, for example, in one margin 56 of the section 46 may be connected to suitable wire leads 93, 99 for connection to the source of electrical heating current.

The other electrical heating structures 47 and 48 may be made in two sections united by splicing like the structure 46 and may be electrically connected to the source of heating current in like manner by wire leads 100, 101 and 102, 103. The particular form of covering shown in the drawings has the relatively wider and tapered heating structures 46, 48 spaced-apart chordwise for disposition at the rearward areas of the leading edge, and has the relatively narrow and rectangular intermediate or frontal heating structure 47 for disposition at the frontal or immediate leading edge region of the airfoil or wing 32. The intermediate or central or frontal heating structure 47 is desirably heated continuously to provide relatively higher temperatures or heat-dissipation for anti-icing heating at the outer surface of the frontal or central heating zone 37 of the covering, where the ice tends to form. first and deposit the thickest, and such continuous heating by the structure 47 maintains a substantial gap in the chordwise continuity of the ice-deposit. Relatively lower temperatures or heat-dissipation may be employed at the desirably cyclically heated rearward heating zones 36, 38 to melt the overlying ice-deposit only at its inner surface nearest the covering to break the adhesive bond therebetween and at the same time produce only a very thin water film between the covering and the overlying ice, whereby the rearward sweep of the airflow effectively removes the remaining rearward ice without permitting the formation of objectionable icedeposits on the more rearward unprotected areas of the wing resulting from the re-freezing of flow-back water from melted ice. The flow-back water from the central or frontal heating zone 37 is normally caught and refrozen on the ice-deposit at the cyclically heating rearward heating zones 36, 38 of the covering.

The fabric and rubber-like parts of the covering including the individual heating structures 46, 47, 48 are desirably united integrally as by vulcanization under heat and pressure. This produces the desired unitary integral covering adapted for fiexure and conformance about and along the leading edge, of the airfoil.

For making an individual electrical heating structure '46, for example, or a section 46a, 46b thereof in accordance with the method steps of the invention, there is provided an elongate cylindrical drum 75 mounted on a shaft 76 journaled in vertical supports 77, 7,8 of a frame .79; The rotatable drum 75 .is power driven by suitable gear mechanism and electrical power means (not shown), which power means may be used to drive a suitable spool-carrying and advancing means (not'shown) to advance a spool 82 having suitable resistance heating wire 64 wound thereon, along the drum at a uniform rate, the spool 8-2 being located at the top of the drum and spaced radially to a side thereof, as shown in Figs. 5 and 6, tension mechanism 81 being provided to effect a tensioned condition of the wire 64 as it is wound on the drum '75. I

The method of making the tapered individual heating structure 46 or section 46a thereof includes the steps of forming areinforcing first ply 59 in a cylindrical shape upon the cylindrical supporting surface of the drum 75, and this is accomplished by wrapping once around the drum 75 a sheet of straight-laid, square woven glass fabric coated on both sides with unvulcanized rubbery polychloroprenecomposition and forming a lap joint'on the ends of the sheet using suitable liquid neoprene cement to hold the joint. Small removable tabs 83, 83 of plastic adhesive tape are utilized to hold the cylindrical reinforcing first ply 59 against shifting axially and circumferentially relative to the drum as shown especially in Fig. 7. Then a plurality of pairs of narrow bands 84, 84 and 85, 85 of non-adhesive thin woven textile fabric, impregnated with starch and known as Holland cloth, are placed on the reinforcing first ply 59 in circumferentially spaced-apart relation with each pair of strips extending in side-by-side abutting relation longitudinally or axially along the cylindrical ply 59 in inclined relation, as shown, or, if desired, in parallel relation to the longitudinal axis thereof, the inclined relationship being used for a tapered construction and the parallel relationship being used for a rectangular construction of the heating structure. The tackiness of the rubber-like coating on-- the first ply is sufiicient to retain the bands in place.

The circumferential spacing and longitudinal disposition of the bands of Holland cloth are established by using a template 86 of suitable transparent, relatively stiff plastic material having approximately the desired shape of the heatingstructure, for example, a tapered shape, and conforming substantially to the curvature of the drum, as shown especially in Figs. 7, 9 and 8. The tapered template 86 is also used for making longitudinally-extending straight pencil lines 87, 88 on the bands of Holland cloth, which pencil lines serve as indicia of the position of each flexible conductor 62, 63.

It will be noted that for the tapered construction of the heating structure the wide end of the tapered template is placed at one end of the cylindrical ply 59 for outlining one heating structure, and then the wide end of the template is placed at the opposite end of the ply to outline the next adjacent heating structure as shown in Fig. 5. This provides complementary tapered areas in the first ply 59 and is advantageous to avoid waste material and to make a plurality of tapered heating structures on the cylindrical drum.

A longitudinally-extending, continuous, stranded and braided wire conductor 62, 63 in flat strip form is next placed face down on each of the Holland cloth bands 84, 85 with the inner margin of the conductor 6-2, 63 at and conforming to the longitudinal straight pencil line 87, 88 on the underlying band. The conductors 62, 63 make feasible the provision of the successive relatively short conductors 62, 62 and 63, 63 in the passages or hollow margins of the heating structure 46 by virtue of severing the continuous conductors in a manner to be described more fully hereinafter. Each continuous conductor 62,63 is desirably held taut along the inclined straight pencil line by means of suitable tabs 91, 92 at each end of the drum; as shown especially in Fig. 7.

A continuous, resistance heating wire 64 having one end thereof secured to the drum by suitable tabs 9.3,. 93 is drawn, off the spool 82 and woundv helically about and along the cylindrical first reinforcing ply 59 and across all the Holland cloth bands 84, 84 and 85,85 and all the continuous conductors 62, 63 in superimposed relation thereto with adjacent convolutions of the heating wire 64 in closely adjacent, spaced-apart relation one to the other and in slightly inclined relation to a plane normal to and intersecting the longitudinal axis of the drum 75. The heating wire 64 is maintained under tension while being wound, and its tensioned condition together with the tackiness of the rubber-like surface of the first ply 59 functions to partially imbed the wire in the rubber-like material and also to retain the wire as wound in the desired helical path. After the desired number of convolutions have been wound about the drum, the wire is cut loose from the spool 82 and the cutend secured by a suitable tab to the drum at the other end thereof.

The tapered template 86 is then placed on the assembled material on the drum 75 between a pair of spacedapart continuous conductors 62, 63 as shown especially in Fig. 7, and the conductors are adjusted to be oppositely inclined relative to the longitudinal axis of the cylindrical first ply 59 and to conform to the longitudinal edges of the tapered template so as to establish the exact tapered and spaced-apart relationship of the conductors and their disposition adjacent the longitudinal margins of the tapered area of the first ply 59. All the convolutions of the heating wires 64 are then soldered at 65 to and entirely across the exposed upper face of each of the conductors to provide good low-loss electrical connection and a secure mechanical attachment of the convolutions to the conductors. It is to be noted that the Holland cloth bands 84, 84 and 85, 85 insulate the underlying rubber-like material against damage from the heat of the soldering operation and restrain the flow of soldering fiux onto the adjacent unprotected rubberlike material where such soldering flux is objectionable because it prevents good bonding and vulcanization.

After completion of the soldering, the soldering flux on the Holland cloth bands and in the space between the immediately adjacent pairs of continuous conductors 62, 63 is removed by a suitable solvent such as carbon tetrachloride, and then the heating wires extending in taut suspended condition across such space between the immediately adjacent pairs of continuous conductors are desirably cemented as by a suitable liquid neoprene cement to the underlying Holland cloth bands, the cement filling the shallow space between the overlying wires and the underlying bands to effect good attachment of the wires to the bands, although for increased clarity such space is shown free of cement in Fig. 11.

A reinforcing second ply 60 of straight-laid, squarewoven glass fabric in strip form coated on both sides with unvulcanized rubbery polychloroprene composition and pre-cut to approximately the tapered shape of the heating structure 46, is then applied and suitably cemented to both the cylindrical reinforcing first ply 59 and the convolutions of heating wire 64 in superimposed relation thereto in each area thereof between adjacent pairs 84 and 85 of the Holland cloth bands with a lateral marginal portion 60a, 60b of the reinforcing second ply extending axially beyond each conductor 62, 63 at each longitudinal margin of said area, the extent of axial overlap of the marginal portions 60a, 60b being sufiicient to accommodate folding such marginal portions about the conductors and heating wires in their overlapped bent condition as shown especially in Figs. 3 and 16. The lateral marginal Portions 60a, 60b overlying and extending laterally beyond the conductors are not adhered to the conductors and heating wires.

After the respective pre-cut reinforcing second plies 60 are attached adhesively as described hereinabove, all the convolutions of the heating wire 64 and the cylindrical first ply 59 are severed along the longitudinal lines 89, 90 of abutment of each pair of Holland cloth bands 84, 85

. so as to provide a plurality of complementary tapered foundational units each having the reinforcing first and second plies 59, 60, a pair of spaced-apart, longitudinally-extendingcontinuous conductors 62, 63, and a plurality of the resistance heating wires 64, 64 extending between and soldered to the pair of conductors. Each of the respective tapered foundational units are assembled in the manner described hereinabove and then all are stripped from the cylindrical supporting surface of the drum 75.

For producing one electrical heating structure of tapered shape, for example, a foundational unit may then be supported on a fiat table top, for example, and the free ends of the cut wires which are attached to the underlying Holland cloth bands and project laterally of the conductors, are then trimmed together with the bands 84, and first ply 59 flush with the outer margin of each braided Wire conductor of the foundational unit, and the remainders of the bands 84, 85 under the conductors are then removed. The lateral marginal portions 60a, 60b of the reinforcing second ply 60 are next folded backwardly and inwardly and held in the folded back condition by several strips 94, 94 of detachable adhesive paper, as shown especially in Figs. 10, l1, l2 and 13. The next step is to sever the marginal portions of the reinforcing first ply 59 immediately underlying the conductors so that the final marginal edges 59a; 59b of the ply 59 are substantially parallel to and spaced inch to inch from the adjacent inner margin of each continuous conductor 62, 63, which spacing represents substantially the thickness of the edge of the first ply 59 and an insulating sheet 61 plus the thickness of a braided conductor. A continuous, thin insulating sheet 61 of resilient all-neoprene material is applied to and throughout the severed reinforcing first ply 59 at the face thereof opposite the heating wires 64, 64, and is adhered to the ply 59 by a suitable liquid neoprene cement, whereby the side margins of the neoprene sheet or layer 61 coincide with the cut-back margins 59a, 59b of the reinforcing first ply 59, as shown especially in Figs. 12 and 13. This completes the steps of making the foundational unit for the heating structure.

For completing the assembly of the heating struc ture 46 a thin separator strip 95 of polyethylene material known as polythene is then applied to the neoprene layer 61 along and spaced at least the width of a conductor, for example, about A in. laterally inward of each side margin of the layer 61 so as to provide a welldefined exposed edge for the subsequent attachment thereto of an insulating tape and the inwardly folded conductor and wires, as shown in Figs. 14 and 15. The tackiness of the neoprene layer 61 retains the polythene strips in place.

A narrow insulating tape 66, 67 madeof two, thin plies of suitable woven fabric coated with rubbery polychloroprene composition on both sides and having sufficient width to wind once fully about a conductor, is then applied and adhered by suitable neoprene rubber cement to each said well-defined exposed marginal portion of the layer 61, and extendslaterally inward in overlying unattached relation to the adjacent polythene strip 95. Each tape 66, 67 has its side margin coinciding with the adjacent edges 59a, 59b of the reinforcing first ply 59 and the neoprene insulating layer 61, as shown especially in Fig. 14.

The next step is to sever each of the continuous con ductors 62, 63 at spaced-apart positions along the same so that the material of the conductor between immediately adjacent reaches of heating wires 64, 64 is removed and the resulting spaces closed by spacers or fillers 96, 96 of rubbery polychloroprene material equivalent in thickness and width to the conductor. The positions of severance of one continuous conductor are in staggered relation to the positions of severance of the other continuous conductor, whereby the plurality of resistance heating wires 64, 64 are, arranged in groups along the tapered foundational unit with the number of wires in each group, desirably equal, such that the individual wires thereof vary only slightly in length and resistance relative to one another despite the convergence of the conductors and are connected in parallel electrically by the conductors and whereby the conductors extend between and connect adjacent groups so as to place them in series electrically. This makes possible effectingsubstantially uniform heat dissipation for each unit of area of the tapered heating zone throughout a group of the heating wires. It will be noted that the severance of the conductors at the spaced-apart positions along the same produces the desired succession of short flexible conductors along the spaced-apart margins of the heating structure.

The succession of flexible conductors 62, 62 and 63, 63 and the exposed end portions of their associated heating wires 64, 64 are bent laterally inward so as to position the soldered portions of the Wires upper or outermost and seat the fiat faces of the conductors directly against the adjacent underlying insulating tape 66 which overlies the insulating sheet 61 and insulating ply 59 covering the adjacent wires 64, 64, as shown especially in Fig. 15, the end portions of the heating wires 64, 64 being bent about what was initially the inner margin of the conductor but which margin, as a result of the bending, is now the outer margin of the conductor in its inwardly bent condition. Next, the narrow two ply insulating tape 66, 67 at each margin has it's free unattached portion folded completely about the conductor and the bent end portions of the heating wires 64, 64 and is attached thereto, as by suitable liquid neoprene cement, in overlying adhered relation to the respective successive conductors and the heating wires, as shown in Fig. 15, to electrically insulate and to mechanically protect the same.

The final steps of making the heating structure 46 are removing the polythene strips 95, applying suitable liquid neoprene cement to the folded-in marginal portions 60a, 60b of the ply 6t and folding outwardly and then inwardly such marginal portions 60a, 60b about the folded two-ply tape 66, 67, the heating wires 64, 64 and the succession of conductors 62, 62, 63, 63 so as to entirely enclose the same in a protective sheath of the fiexible dielectric rubber and fabric material, as shown especially in Fig. 16. The heating structure is then ready for assembly and vulcanization with the other parts of the covering described hereinabove. The heating structure 46 is disposed with the heating wires uppermost for positioning nearest to the outer layer 45 of the covering for facilitating rapidity of transmission of heat to the outer surface of the covering while minimizing heat loss to the underlying structure of the air foil, and for maintaining effective ice-removal temperatures at such outer surface. 7

The construction and arrangement of the heating structure 46 or section 46a thereof made in accordance with the method steps described hereinabove produces heating by the resistance wires 64, 64 from edge or margin to edge or margin of the structure; and since the dielectric material enclosing the heating wires and conductors at the margins is relatively thin, there is little or no gap in the heating between adjacent margins of adjacent heating structures in side-by-side adjoining relation. This is important for continuity of heating throughout the heating zones 36, 37, 38 of the covering and for avoiding residual ice along the adjoining margins of adjacent heating zones. The resilient construction of the heating structures and the covering is advantageous for absorbing the impact of stones and for resisting breakage of the heatig wires under such impact of stones, while at the same time the particular arrangement of the heating structure and the heating wires minimizes the effect of the breakage of one or more wires upon the heating of the covering as a whole. Also, the hereinabove de scribed construction makes feasible the provision of a trapezoidal shape of the heating structure or covering and its heating grid with attendant independent mounting and smooth fit of the structure or covering as an integral unit on the airfoil such as a wing of gradually decreasing thickness along its leading edge from its inboard end to its outboard end.

Variations may be made without departing from the scope of the invention as it is defined in the following claims:

I claim:

1. A method of making an electrical heating structure for a protective covering suitable for use on airfoils and other aircraft surfaces, which method comprises the steps of forming and temporarily retaining a first ply of elastic dielectric material in a cylindricalv shape upon a cylindrical supporting surface, laying and temporarily retaining flexible conductors upon and longitudinally along the cylindrical first ply while arranging the conductors in longitudinal pairs of adjacent spaced-apart substantially parallel conductors at circumferentially spaced-apart positions in the first ply with said pairs each so inclined relative to the longitudinal axis of the cylindrical first ply as to provide complementary tapered areas in the first ply between the spaced pairs of conductors, winding and retaining a continuous electrical resistance heating wire helically about said cylindrical first ply and across all said conductors in superimposed relation thereto with the convolutions of wire in closely spaced-apart substantially parallel relation one to the other, soldering all said convolutions of wires to all said conductors, applying and adhering a second ply of elastic dielectric material in tapered strip form to said first ply and to said convolutions of wire in superimposed relation thereto throughout one of said tapered areas with longitudinal marginal portions projecting circumferentially beyond the outer margins of the conductors at the tapering side boundaries of the last said tapered area in unattached relation to the underlying first ply, conductors and heating wires severing said convolutions of wires and said first ply along said outer margins of the last said conductors to provide an individual foundational unit of tapered form having the united first and second plies, a pair of laterally spaced conductors and a plurality of heating wires sandwiched between said plies and extending between the conductors, stripping said foundational unit from said cylindrical supporting surface, severing the first ply of said unit along longitudinal lines adjacent and spaced from the inner margins of said pair of conductors of said unit to provide exposed end portions of the heating wires, severing and insulating each of the last said pair of conductors at a plurality of positions therein so as to arrange the heating wires of the unit in groups connected in series electrically, bending and retaining the exposed end portions of the heating wires with their associated conductors laterally inwardly about the severed edges of said first ply of the unit, and folding and adhering said marginal portions of said second ply laterally inwardly about said severed conductors and the exposed end portions of said heating wires in their bent condition to enclose the same.

I 2. The method of making trapezoidal electric heating structures comprising the steps of mounting and retaining a cont1nuous band of fabric sheet material treated with dielectric rubber upon a cylindrical drum, mounting and retaining a plurality of elongated conductors upon the fabric band at a face thereof so as to form a plurality of axially extended closely spaced paired conductor rows that divide the fabric band into trapezoidal areas between the paired rows of conductors, circumferentially wrapping resistance wire in convolutions about and along the fabric band and conductors and attaching the wire thereto, adhesively securing an individual sheet including dielectric rubber to and in contact with said face of the fabric band and to said wire convolutions substantially throughout each of said trapezoidal areas with marginal portions of the individual sheet extending circumferentially beyond a pair of conductors of a trapezoidal area in unattached relation to the underlying fabric band and conductors and wire convolutions, cutting the wire convolutions and fabric band between the spaced conductors of each row to form individual composite trapezoidal elements, cutting away said fabric sheet material directly underlying the conductors of each trapezoidal element so as to clear said conductors and expose short end lengths of all said wires along the conductors of the trapezoidal element, bending the exposed end lengths of wire of each trapezoidal element in a manner to place said conductors thereof against dielectric rubber at the other face of the fabric sheet material adjacent the cut edges thereof, and folding and securing said marginal portions of said individual sheet about the bent end lengths of the wires and said conductors of each of said trapezoidal elements.

3. A method of making a plurality of foundational heating units of tetragonal form in plan for electrical heating structures, which method comprises the steps of forming and temporarily retaining a first ply of fabric reinforced elastic dielectric material in a cylindrical shape upon a cylindrical supporting surface, laying and temporarily retaining flexible conductors upon and longitudinally along the cylindrical first ply While arranging the conductors in longitudinal pairs of adjacent spaced-apart substantially parallel conductors at circumferentially spaced-apart positions in the first ply with said pairs each so disposed relative to the longitudinal axis of the cylindrical first ply as to provide tetragonal areas in the first ply between the spaced pairs of conductors, winding and retaining a continuous resistance heating wire helically about said cylindrical first ply and across all said pairs of conductors in superimposed relation thereto with the convolutions of wire in closely spaced substantially parallel relation one to the other, mechanically and electrically connecting each of said convolutions of wire to each of said conductors, applying and adhering individual second plies of fabric reinforced elastic dielectric material in tetragonal strip form to and about said cylindrical first ply and to said convolutions of wire in superimposed relation thereto so that one individual second ply is disposed at and adhesively attached to each of said tetragonal areas in said first ply with longitudinal marginal portions of the individual second ply projecting substantially and circumferentially beyond a pair of longitudinal conductors at the spaced margins of its associated tetragonal area in unattached relation of the underlying first ply, conductors and the heating wires, severing all said convolutions of wire and said cylindrical first ply along the outer margins of the last said pair of longitudinal conductors of each of said tetragonal areas to provide a plurality of integral individual tetragonal foundation units each having the united first and second plies, a pair of laterally spaced conductors and a plurality of heating wires sandwiched between said plies and extending between the conductors, stripping all said integral individual foundational units from said cylindrical supporting surface, severing the first ply of each of said units along longitudinal lines adjacent and spaced from the inner margins of the said pair of laterally spaced conductors of the unit to provide exposed end portions of the heating wires, bending and retaining the exposed end portions of the heating wires of each of said units inwardly about the severed edges of said first ply of the unit, and folding and adhering said marginal portions of said second ply of each of said units laterally inward about the conductors and the exposed end portions of the heating wires in their bent condition to enclose the same.

4. The method of making a tetragonal electric heating structure comprising the steps of retaining a flexible fabric sheet including elastic dielectric material in an arcuate shape, retaining a plurality of elongated conductors upon the fabric sheet so as to provide at least one pair of axially extending spaced conductors that divide the arcuate fabric sheet into at least one tetragonal area between said pair of conductors, wrapping resistance heating wire in convolutions about and along said arcuate fabric sheet and said conductors and attaching the wire thereto, securing an individual sheet including elastic dielectric material to and in contact with a face of the arcuate fabric sheet and to the wire convolutions substantially throughout said tetragonal area with marginal portions of the said individual sheet extending circumferentially beyond the said pair of conductors of said tetragonal area in unattached relation to the underlying fabric sheet and conductors and wire convolutions, cutting the wire convolutions and fabric sheet at the outer margins of said pair of conductors to form an individual composite tetragonal element, cutting away that part of said fabric sheet directly underlying said conductors of the tetragonal element so as to clear said conductors and expose short end lengths of all said wires along the conductors of the tetragonal element, bending the exposed end lengths of wire of the tetragonal element in a manner to place said conductors thereof against dielectric material at the other face of the fabric sheet adjacent the cut edges thereof, and folding and securing said marginal portions of said individual sheet about the bent end lengths of the wires and said conductors of the tetragonal element.

References Cited in the file of this patent UNITED STATES PATENTS 1,519,522 Weigel Dec. 16, 1924 2,283,880 Heller May 19, 1942 2,503,457 Speir et al Apr. 11, 1950 2,575,987 York et al Nov. 20, 1951 2,619,580 Pontiere Nov. 25, 1952 2,657,161 Luitwieler Oct. 27, 1953 FOREIGN PATENTS 462,237 Canada Jan. 3, 1950 

